Method and apparatus for carrying out underwater well operations

ABSTRACT

Method and apparatus for carrying out operations with respect to an underwater well wherein a cable is secured between the well and a platform mounted on a floating vessel over the well site. The platform is mounted for vertical movement relative to the vessel and is continuously biased in an upward direction during said relative movement to maintain the drill pipe under tension and compensate for up and down movement of the vessel.

United States Patent Vincken et a1. 45 Aug, 8, 1972 [54] METHOD ANDAPPARATUS FOR [56] Reierences Cited CARRYING OUT UNDERWATER WELLOPERATIONS 3 421 581 STAIES [721 Inventors mm We; 325661608 34371m'filiIIIIIIIIIIIIIIIIQriiEZ AWN, 2,809,435 10/1957 Nicholson et al.....1-'/s/27 ux Petroleum Company Limited, Sena, State of 3mm; PMHoekmga, Primary Examiner-Jacob Shapiro Y Care! van Bylandwan TheAttorney-Harold L. Denkler and Theodore E. Bieber Hague, all ofNetherlands [22] Filed: Sept. 15, 1970 ABSTRACT 21 L N 72 Method andapparatus for carrying out operations 1 App 0 325 with respect to anunderwater well wherein a cable is secured between the well and aplatfonn mounted on [52] US. Cl ..6l/46.5, 166/.5 a floating vessel overthe well site. The platform is [51] Int. Cl.....B63b 35/44, B63b 43/06,E2lb 31/05 mounted for vertical movement relative to the vessel [58]Field of Search ..6l/46.5, 69; 175/27, 7; and is continuously biased inan upward direction durl66/.5; 114/.50 ing said relative movement tomaintain the drill pipe under tension and compensate for up and downmovement of the vessel.

8Claim2DrawingFigures PATENTEDAus 8 1912 SHEET 2 BF 2 SENSING DEVICEDEVICE 4 (25 SENSING PUMP 3| r33 SERVQ VALVE MECHANISM FIG.2

INVENTORS: I

LEONARDUS M. J. VINKEN PAUL F. AGERBEEK PIET E. HOEKENGA ATTORNEY METHODAND APPARATUS FOR CARRYING OUT UNDERWATER WELL OPERATIONS BACKGROUND OFTHE INVENTION I l Field of the Invention This invention relates to amethod and to an apparatus for carrying out well operations under thesurface of a body of water.

2. Description of the Prior Art In deep water, floating structures aresometimes used, from which underwater operations are carried out, e.g.,on an oil or gas well. Difficulty is often experienced if the floatingstructure tends to heave and to move vertically because of waves, sinceit is rather disturbing when it is necessary to carry out from thefloating structure operations on a fixed submerged object such as an oilwell, a gas well or a pipeline.

SUMMARY OF THE INVENTION It is an object of the invention to provide amethod and an apparatus having the advantage that the influence of theheave and of the vertical movement of the floating structure iscompensated for, so that they no longer disturb the operations to becarried out under water.

The method according to the invention therefore comprises securing atleast one flexible cable at one end thereof to a fixed point below thesurface of the body of water, securing the other end of the cable tocable-engaging means on a platform which is secured to a vessel suchthat it is movable at least in substantially vertical direction withrespect to the vessel, applying a substantially constant force betweenthe platform and the vessel such that the force acting on the platformis directed upwards and tensioning the cable such that the load on theflexible cable is equal to the force applied between the platform andthe vessel minus the weight of the platform and any equipment thereon,and carrying out operations from the platform below the surface of thebody of water.

A floating structure for carrying out the method according to theinvention comprises a platform so secured to a floating vessel that itis movable relatively to the vessel, at least in substantially verticaldirection with respect to the vessel, means on the structure forapplying a force between the platform and the vessel, and cable-engagingmeans on the platform for engaging a cable which is suitable to bearranged substantially vertical between the cable-engaging means and afixed point below the surface of the body of water on which thestructure may float.

The invention may be carried into practice in various ways but twospecific embodiments will now be described by way of example withreference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a diagrammatic sideview ofan embodiment of a floating structure suitable for carrying out themethod according to the invention.

FIG. 2 is a diagrammatic sideview of partly in block form of a modifiedembodiment of a floating structure suitable for carrying out the methodaccording to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1 and 2 show structures 1and la floating on a body of water 2 having a surface 3, whichstructures 1 and la comprise vessels 4 and 4a and platforms 5 and 5a,respectively. The platforms 5 and 5a are so secured to the floatingvessels 4 and 40, for example, by suitable vertical guide means (notshown), that they are vertically movable relatively to the vessels 4 and4a.

In the embodiment shown in FIG. 1, cables 6 are secured to the platform5, the other end of each cable 6 being provided with a weight 7 and eachcable 6 being led over a pulley 8. Each pulley 8 is rotatably secured toa pillar 9 which is supported by the vessel 4. Each pulley 8 is arrangedat a level above the highest possible position of the platform 5relatively to the vessel 4. The platfonn 5 is in its highest positionrelative to the vessel 4, if the weights 7 are supported by the deck ofthe vessel 4.

The vessel 4 is positioned above a submerged well head 10 by means knownper se such as anchors and anchor cables. The wellhead l0 rests with itsbase 11 on the bottom 12 of the body of water 2. This base 11 isprovided with connecting means 13 for connecting the lower ends ofcables 14 to the base. The upper ends of these cables 14 are attached tothe cable engaging means 15 of winches (not shown) which are mounted onthe platform 5. The cables 14 pass through openings 16 provided in theplatform 5 and through a U-shaped depression 17 of the side wall of thevessel 4.

Maintenance or other operations can be carried out in the well via thewellhead 10 by means of a cable 18 passing through an opening 19 of theplatform 5 and being guided over a pulley 20 rotatably secured to apillar 21 which is supported by the platform 5. One end of the cablecarries a tool (not shown) for performing operations within the well,and the other end of the cable 18 is wound around a drum 22 which can beactuated by means of a driving mechanism (not shown). The drum 22 aswell as the driving mechanism are mounted on the platform 5.

The floating structure as shown in FIG. 1 is used as follows:

One end of each cable 14 is latched on to the base 1 I of an oil well(not shown) in a manner well known in the art, such as by using divers,guiding cables or a remotely operated propulsion device having atelevision camera. Although the connecting means 13 have been shown inthe drawing in the form of a hook, it will be understood that the otherconnecting means may be used as well such as connections known in theart which can be remotely controlled and which include emergencyreleases.

The upper ends of the cables 14 pass together through the depression 17of the vessel 4 and separately through holes 16 provided in the platform5. Each cable 14 is connected to the cable-engaging means 15 of winches(not shown). By activating the winches, the cables 14 will be tensionedand the platform 5 will move downwards against the action of the weights7. The total load on the cables 14 will then be equal to the total forceapplied between the platform 5 and the vessel 4, which force is suppliedby the weights 7 suspended from the cables 6, minus the weight of theplatform and any equipment thereon. If the weights 7 have been broughtin an intermediate position between the extreme positions which they canreach, the winches are stopped and blocked against further rotation.

If the vessel 4 moves vertically because of waves, the platform 5 willremain in its position relative to the bottom 12 of the body of water 2.The vessel 4 on moving upwards displaces the pillars 9 and the pulleys 8upwardly. Since the platform 5 is retained by the cables 14, it cannotmove upwards and the pulleys 8 are rotated and the weight 7 displaced.

The platform 5 also remains stationary and at the same distance from thebottom 12 when the vessel 4 moves downwards, since the cables 14 arekept taut under influence of the weights 7. During this downwardmovement of the vessel 4, the weights 7 are displaced and the pulleys 8rotate in a direction opposite to the direction of rotation during theupward movement of the vessel 4.

From the above, it will be clear that, irrespective of the waves, theplatform 5 will always remain at the same distance from the bottom 12and that the cables 14 will always be kept under tension.

From the platform 5, drilling or wireline operations can be carried out.The platform 5 can be provided with conventional equipment for carryingout wireline operations, for example, equipment as shown in FIG. 1 andindicated by the numbers 18, 19, 20, 21 and 22. Since this equipmentdoes not form part of the invention, it will not be discussed in detail.The wellhead is provided with a high-pressure seal through which thecable 18 can pass, and this cable as well as the tool (not shown)suspended from this cable can be entered into the well via the wellhead10 by means of diver operation.

The tensioning of the cables 14 need not necessarily occur by means ofthe winches (not shown),of which the cable-engaging means form part. Ifdesired, the winches may be used only for storage of the cables 14, andpaying these cables out to a length at which they are connected at theirlower ends to the cable-connecting means 13. Thereafter, the cables canbe tensioned by exerting a force between the platform 5 and the vessel 4to move the platform in an upward direction. This may be reached bykeeping the weights 7 in a certain position during the placement of thecables 14, and thereafter leaving the weights 7 free. It will beunderstood that such operation is only possible if there are no, or onlyvery small waves at the water level 3.

Emergency controls (not shown) are provided to stop the platform 5 fromshooting upwardly in the event of one or both of the cables 14 breakingor becoming loosened.

The embodiment as shown in FIG. 2 is provided with a hydraulic systemfor moving the platform 5a relatively to the vessel 4a instead of thesystem comprising weights 7 as shown in FIG. 1. It is remarked, however,that instead of weights or a hydraulic system it is also possible to usea pneumatic, hydro-pneumatic, or an electrical system, if desired. Onthe vessel 4a, a number of (for example, four) hydraulic cylinders 23 isarranged in each of which a plunger or piston (not shown) is presentwhich is axially movable in the cylinder 23. To each plunger or piston abar or rod 24 is secured, of which the upper end is connected to theplatform 5a. The connection of the bars or rods 24 with the platform maybe direct or through a gimbal support. The axial (vertical) movement ofthe bars or rods 24 may be controlled by any suitable control systemwhich senses the deviation or any derivative of the deviation of theplatform 5a from the desired horizontal plane. In the system as shown inFIG. 2, the deviation of the platform 50 from the desired horizontalplane is sensed by a sensing device 25. From the sensing device 25signals can be transmitted through a line 26 to a controller 27. Thetension in the cables which are at their upper ends connected to thecableengaging means can be sensed in a manner known per se by sensingdevices 28 which are each connected toone of the cylinders 23 by lines29. .Each sensing device 28 is connected to the controller 27 by a line30 through which signals can be transmitted from the sensing device tothe controller. The controller 27 can process the signals, received fromthe sensing devices 25 and 28, and can decide on the hydraulic pressurerequired in the cylinders 23 to keep the platform 5 in the desiredhorizontal plane and to keep the cables 140 under the proper load. Thecontroller 27 can then regulate the pressure in the cylinders 23accordingly, e.g., by operating three-way control valves 31 and 32 bysending through lines 35 signals to servo-valve mechanisms 33 and 34.The valves 31 and 32 are arranged respectively in lines 36 and line 37which each connects the pressure side of a liquid-pump 38 with acorresponding cylinder 23. The suction side of the pump 38 is connectedthrough a line 39 to a tank 40 containing a suitable liquid, forexample, oil. A drain line 41 connects the valve 31 with the top of thetank 40 and a drain line 42 connects the valve 32 to the drain line 41.By properly positioning the three-way valves 31 and 32, liquid can bepumped from the tank 40 to the cylinders 23 and/or be drained from thecylinders 23 so that the proper pressure will be obtained in each of thecylinders 23. By means of the hydraulic system as described above theplatform 5a will, irrespective of the waves, always remain in thedesired horizontal plane and the cables 14a will always be kept underthe required load.

On the platform 5a can be located conventional equipment for carryingout drilling operations, for example, a derrick 43, a hoist 44, a cable45, a traveling block 46, and a crown block 47.

The communication with a well via a (not shown) submerged wellhead takesplace via a pipe string 49 which at its lower end is connected to thewellhead and at its upper end passes through an opening 50 in theplatform 5.

The operation of the embodiment as shown in FIG. 2 is as follows:

When the vessel 4a has been located above the well in which maintenanceoperations are to be carried out, the cables 14a are operativelypositioned with their lower ends connected to the wellhead or a point inthe neighborhood of this wellhead as described above with respect to theembodiment of FIG. 1. This point is a fixed point located below thesurface 3 of the body of water 2. By actuating the cable engaging means15a, the cables 14a are tensioned, and, since the load on the cables issensed by the sensing devices 28, the cylinders 23 are controlled suchthat the desired load is exerted in the cables 14a. It will beunderstood that by a suitable manipulation with the cable-engaging means15a, a desired neutral position of the rods 24 relative to the cylinders23 will be obtained.

Tensioning of the cables 14a during wave action exerted on the vessel 4awill result in immobility of the platform 5a relative to the bottom ofthe body of water, and consequently, relative to the wellhead of -a wellin which operations are to be carried out. in this position of theplatform 5a the tube string 49 is lowered from the platform to thewellhead and connected thereto either by diver assistance or by means ofautomatic or remotely controlled coupling equipment known in the an.

The upper end of the tube string 49 is immobile with respect to theplatform 5a, notwithstanding the wave action exerted on the vessel 4a,and may be coupled to the platform 5a by a clamping means 51. Suchclamping means are well known and need no further description. Byactuating the cable-engaging means 15a, the load on the cables 14a maynow be partly removed from these cables to the pipe string 49. This,however, is not necessary, and the operations to be carried out in thewell by means of the string 48 suspended from the hook of the travelingblock 46 may be carried out with either the cables 14a and/or the pipestring 49 under load. It will be understood that when the string 49 isloaded by the action of the hydraulic system, the load exerted thereonwill have to be sensed and the data obtained to be fed to the controller27.

The wellhead, or other submerged equipment on which operations are to becarried out by means of the method and/or apparatus according to theinvention need not be located on the bottom of the body of water, butmay also be located above or below this bottom. It is, however, reallyessential that the flexible cables 14a or the pipe string 49 areconnected at their lower ends to a point below the water level, whichpoint has a constant distance with respect to the bottom of the body ofwater.

it is to be noted that instead of the weights or the hydraulic system asdescribed above a pneumatic,

hydro-pneumatic, or electric system may also be used. It is, forexample, possible to keep the pressure in each cylinder 23 substantiallyconstant in all positions of the platform 5a by connecting each cylinder23 to a corresponding pressure tank (not shown), containing gas andhaving a volume which is large relative to the volume of thecorresponding cylinder 23.

Furthermore, attention is drawn to the fact that instead of one pipestring 49 as shown, it is possible to use a plurality of pipe stringswhich are parallelly or concentrically arranged.

It will be understood that although the wave compensated platform, asdescribed in the embodiment of the invention according to FIGS. 1 and 2is mounted above the vessel, the invention is not limited thereto.Similar results will be obtained when securing the platform to one ofthe sides of the vessel in such a manner that it is movable at least ina substantially vertical direction with respect to the vessel.

The embodiments as shown in FIG. 1 may, if desired, also be combinedwith a pipe string 49 as described with reference to F IG. 2.

, 6 The winch 22 (FIG. 1) need not be placed on the platform 5, but maybe mounted on the vessel 4 at a sufficient distance from the platform.The cable 18 is then guided from the winch, over a guide pulley mountedon the platform 4, and over the pulley 20. A similar arrangement may bemade in the embodiment as shown in FIG. 2.

We claim as our invention:

1. A method of carrying out operations on a well having a wellheadpositioned under the surface of a body of water, comprising the stepsof:

arranging a platform on a floating vessel 'such that the platform ismoveable at least in a substantially vertical direction with respect tothe vessel;

positioning the vessel above the wellhead;

securing one end of each of a plurality of flexible cables to at leastone fixed point below the surface of the body of water;

securing the opposite end of each of the cables to winch meansoperatively connected to the platform;

applying between the platform and the vessel a substantially constantforce upwardly directed with respect to the platform and of magnitudesufficient to move the platform and any equipment thereon upwardly withrespect to the vessel;

after securing the cables to the fixed point, actuating the winch meansto adjust the length of the cables to a length selected to maintainthe'platform at a selected distance from the bottom of the body ofwater; the selected distance being one from which the platform is freeto move vertically both upwardly and downwardly with respect to thevessel;

tensioning the cables to prevent upward movement of the platform withrespect to the vessel under the applied substantially constant forcewhen the vessel is at rest; and

carrying out underwater operations from the platform.

2. The method according to claim 1, wherein the force between theplatform and the vessel is created by means of at least one weight.

3. The method according to claim 1, wherein the force between theplatform and the vessel is created by a piston arranged in a fluidcylinder.

4. An apparatus for carrying out operations at offshore well locationscomprising:

a base fixedly positioned adjacent the well below the surface of thebody of water;

a floating vessel,

a work platform operatively connected to said floating vessel so thatsaid work platform is vertically moveable with respect to said floatingvessel,

means for applying vertically directed force between said work platformand said floating vessel,

a plurality of flexible cables operatively connected between said workplatfonn and said base,

cable-engaging means on said work platform for engaging each of saidcables, and

wench means mounted on the platform, one winch means operativelyassociated with each of said cable-engaging means for adjusting thelength of said cables and thereby adjusting the tension in said cables.

5. The structure as claimed in claim 4, in which the means for applyinga vertically directed force to the platform comprises at least one cablehaving one end secured to the platform and the other end to a weight,the cable being led over a pulley which is arranged on the structure ata level above the highest position of the platform relatively to thevessel.

6. The structure as claimed in claim 4, in which the means for applyingan upwardly directed force to the platform comprises at least onecylinder arranged on the structure, a piston arranged in the cylinder, aconnection between the piston and the platform, a pressure sourcecommunicating with the cylinder, and means for

1. A method of carrying out operations on a well having a wellheadpositioned under the surface of a body of water, comprising the stepsof: arranging a platform on a floating vessel such that the platform ismoveable at least in a substantially vertical direction with respect tothe vessel; positioning the vessel above the wellhead; securing one endof each of a plurality of flexible cables to at least one fixed pointbelow the surface of the body of water; securing the opposite end ofeach of the cables to winch means operatively connected to the platform;applying between the platform and the vessel a substantially constantforce upwardly directed with respect to the platform and of magnitudesufficient to move the platform and any equipment thereon upwardly withrespect to the vessel; after securing the cables to the fixed point,actuating the winch means to adjust the length of the cables to a lengthselected to maintain the platform at a selected distance from the bottomof the body of water; the selected distance being one from which theplatform is free to move vertically both upwardly and downwardly withrespect to the vessel; tensioning the cables to prevent upward movementof the platform with respect to the vessel under the appliedsubstantially constant force when the vessel is at rest; and carryingout underwater operations from the platform.
 2. The method according toclaim 1, wherein the force between the platform and the vessel iscreated by means of at least one weight.
 3. The method according toclaim 1, wherein the force between the platform and the vessel iscreated by a piston arranged in a fluid cylinder.
 4. An apparatus forcarrying out operations at offshore well locations comprising: a basefixedly positioned adjacent the well below the surface of the body ofwater; a floating vessel, a work platform operatively connected to saidfloating vessel so that said work platform is vertically moveable withrespect to said floating vessel, means for applying vertically directedforce between said work platform and said floating vessel, a pluralityof flexible cables operatively connected between said work platform andsaid base, cable-engagIng means on said work platform for engaging eachof said cables, and wench means mounted on the platform, one winch meansoperatively associated with each of said cable-engaging means foradjusting the length of said cables and thereby adjusting the tension insaid cables.
 5. The structure as claimed in claim 4, in which the meansfor applying a vertically directed force to the platform comprises atleast one cable having one end secured to the platform and the other endto a weight, the cable being led over a pulley which is arranged on thestructure at a level above the highest position of the platformrelatively to the vessel.
 6. The structure as claimed in claim 4, inwhich the means for applying an upwardly directed force to the platformcomprises at least one cylinder arranged on the structure, a pistonarranged in the cylinder, a connection between the piston and theplatform, a pressure source communicating with the cylinder, and meansfor controlling the pressure in the cylinder.
 7. The structure asclaimed in claim 6 in which a system is present for keeping the pressurein the cylinder substantially constant in all positions of the platformrelatively to the vessel.
 8. The structure as claimed in claim 7, inwhich the system for keeping the pressure in the cylinder constantcomprises a pressure tank containing gas and having a volume which islarge relative to the volume of the cylinder, and a fluid connectionbetween the cylinder and the pressure tank.